Micro-electro-mechanical system device, out-of-plane sensor and method for making micro-electro-mechanical system device

ABSTRACT

The present invention discloses a micro-electro-mechanical system (MEMS) device, comprising: a mass including a main body and two capacitor plates located at the two sides of the main body and connected with the main body, the two capacitor plates being at different elevation levels; an upper electrode located above one of the two capacitor plates, forming one capacitor therewith; and a lower electrode located below the other of the two capacitor plates, forming another capacitor therewith, wherein the upper and lower electrodes are misaligned with each other in a horizontal direction.

BACKGROUND

1. Field of Invention

The present invention relates to a micro-electro-mechanical system(MEMS) device, in particular an out-of-plane sensor, and a method formaking the MEMS device, which are less affected by the stress caused bythe manufacturing process.

2. Description of Related Art

An out-of-plane sensor senses the capacitance variation resulting from achange of the distance between two electrodes, and generates acorresponding signal. Such out-of-plane sensor may be used in, e.g., anaccelerometer. Related art can be found in, e.g., U.S. Pat. Nos.6,402,968; 6,792,804; 6,845,670; 7,138,694; and 7,258,011.

The above-mentioned prior art discloses either single-capacitorstructure occupying a large area, or differential capacitor structuremade by wafer bonding. The former consumes too much area, while thelatter requires a complicated process that is not compatible withstandard CMOS process.

Therefore, it is desired to provide an out-of-plane sensor with reducedarea, which can be manufactured by standard CMOS process and lessaffected by the stress caused by the manufacturing process.

SUMMARY

In view of the drawback of the prior art, it is a first objective of thepresent invention to provide a MEMS device, in particular anout-of-plane sensor, which is less affected by the stress caused by themanufacturing process.

It is a second objective of the present invention to provide a methodfor making a MEMS device such as an in-plane sensor.

In accordance with the foregoing and other objectives of the presentinvention, and from one aspect of the present invention, the presentinvention discloses a MEMS device, comprising: a mass including a mainbody and two capacitor plates located at the two sides of the main bodyand connected with the main body, the two capacitor plates being atdifferent elevation levels; an upper electrode located above one of thetwo capacitor plates, forming one capacitor therewith; and a lowerelectrode located below the other of the two capacitor plates, forminganother capacitor therewith, wherein the upper and lower electrodes aremisaligned with each other in a horizontal direction.

In the MEMS device, preferably, the mass further includes two outer massparts connected respectively with the two capacitor plates at the outerside of each of the two capacitor plates.

In another aspect of the present invention, the present inventiondiscloses an out-of-plane sensor, comprising a plurality of MEMSstructure units, each of the MEMS structure unit including: a massincluding a main body and two capacitor plates located at the two sidesof the main body and connected with the main body, the two capacitorplates being at different elevation levels; an upper electrode locatedabove one of the two capacitor plates, forming one capacitor therewith;and a lower electrode located below the other of the two capacitorplates, forming another capacitor therewith, wherein the upper and lowerelectrodes are misaligned with each other in a first horizontaldirection.

In the out-of-plane sensor, preferably, the mass further includes twoouter mass parts connected respectively with the two capacitor plates atthe outer side of each of the two capacitor plates.

The plurality of MEMS structure units in the out-of-plane sensor may bearranged in various layouts; in one example, at least two MEMS structureunits are orthogonal to each other in a horizontal plane.

In the out-of-plane sensor, preferably, a continuous length of the mainbody in at least one horizontal direction is less than a predeterminedlength limit, e.g., 60 μm-100 μm.

In another aspect of the present invention, the present inventiondiscloses a method for making a MEMS device, comprising: providing asubstrate; depositing and patterning at least a contact layer, a metallayer and a via layer, the contact layer, metal layer and via layer as awhole include a to-be-etched region therein; and removing theto-be-etched region to form a MEMS device as described in the above.

In the method for making a MEMS device, preferably, the step of removingthe to-be-etched region includes: first etching the to-be-etched regionby an anisotropic reactive ion etch, and then etching the to-be-etchedregion by hydrogen fluoride vapor etch or buffered oxide etch.

It is to be understood that both the foregoing general description andthe following detailed description are provided as examples, forillustration and not for limiting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings.

FIG. 1 shows a structure embodiment according to the present invention.

FIG. 2 explains how the present invention reduces the impact of theresidual stress.

FIGS. 3-5 show several other structure embodiments of the presentinvention.

FIG. 6 shows, as an example, how the top view may look like from the A-Asection of FIG. 5.

FIG. 7 schematically shows a top view of the MEMS structure unit 100according to the present invention.

FIGS. 8-10 show three layout embodiments of the present invention.

FIGS. 11-14 show a process embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings as referred to throughout the description of the presentinvention are for illustration only, but not drawn according to actualscale.

Referring to FIG. 1, in one embodiment of the present invention, theMEMS structure 100 of an out-of-plane sensor includes a mass 10, a lowerelectrode 20 and an upper electrode 30. In the x-direction of thefigure, the lower electrode 20 and the upper electrode 30 are misalignedwith respect to each other. One differential capacitor is formed betweenthe mass 10 and the lower electrode 20, and another differentialcapacitor is formed between the mass 10 and the upper electrode 30. Whenthe mass 10 moves along the z-direction of the figure, the sensitivityof the sensor is double, because the variation of the capacitance istwice of the variation of the vertical distance. For convenience incapacitance calculation, the vertical distance between the mass 10 andthe lower electrode 20, and the vertical distance between the mass 10and the upper electrode 30 should preferably be the same, but notnecessarily so. In the latter case, the capacitance calculation shouldtake the difference between the distances into consideration.

One merit of the structure as shown is that it can reduce the impact ofstress. Referring to FIG. 2, the mass 10 may be divided into five parts,i.e., the main body 13, two outer mass parts 11 and 15, and twostructure parts 12 and 14 acting like capacitor plates. The twocapacitor plates 12 and 14 are located at different elevation levels (inz-direction), forming two capacitors with the upper and lower electrodes30 and 20, respectively. If the structure part 12 is only connected withthe main body 13, but not with the outer mass part 11 (that is, if theouter mass part 11 does not exist), then bending may occur as shown bythe dash-line block due to stress generated in the manufacturingprocess. Likely, if the structure part 14 is only connected with themain body 13, but not with the outer mass part 15 (that is, if the outermass part 15 does not exist), then bending may also occur as shown bythe dash-line block. However, although both the structure parts 12 and14 bend upwardly, their relationships with the corresponding upper andlower electrodes 30 and 20 are changed asymmetrically. In other words,the process may impact the structure, and leads to inaccuracy incapacitance calculation. In the present invention, since there are outermass parts 11 and 15 provided to connect the outer sides of thestructure parts 12 and 14, the likelihood of bending can be greatlyreduce. Nevertheless, such outer mass parts 11 and 15 are onlypreferred, but not necessary to the present invention. That is, the mass10 can include just the main body 13 and two structure parts 12 and 14,without one or both of the outer mass parts 11 and 15. In this case thelikelihood of bending should be solved by other means.

Under the same spirit of the present invention, the structure of FIG. 1may be modified in various ways, and the number of layers may bedifferent. The key features are that (1) the upper and lower electrodes30 and 20 are misaligned in x-direction, and (2) at least one side ofeach of the capacitor plates is connected with a relatively larger massbody; preferably, both sides of the capacitor plates are connected withrelatively larger mass bodies. FIGS. 3-5 show several more examples.Note that as shown in FIG. 4, the upper and lower electrodes 30 and 20do not require to have the same cross sectional structure, because whateffectively produce the capacitance are the lower surface of the upperelectrode 30, the upper surface of the lower electrode 20, and thedistances between the mass 10 and them. Moreover, as shown in FIG. 5,the two capacitor plates 12 and 14 may be connected with the other parts11, 13 and 15 only at one or several connection points, instead ofintegrated as a whole body. For example, as shown by the dash lines inFIG. 5, they may be connected with one another in only one of theinterconnection layers, from cross-sectional view. The dash lines inFIG. 5 imply that the connection points are not at the same crosssection as the rest of the figure. Or, they may be connected with oneanother only at one or several connection points, from top view. Forexample, the top view taken from the A-A line of FIG. 5 may be as shownin FIG. 6.

The MEMS structure 100 shown in FIGS. 1-5 may be taken as one MEMS unit,having a top view as shown in FIG. 7. The lower electrode 20 is shown bydash lines to imply that it is located beneath the mass 10. Multiplesuch MEMS units may be grouped together, with properly arranged layout,to increase the accuracy of the overall device. FIG. 8 shows an example,wherein multiple MEMS units are arranged by the layout as shown. Notethat the upper-right and lower-left MEMS units are rotated 90°, tocancel the imbalance cause by the stress in x and y directions, so thatthe capacitance measured by the overall device is more accurate. FIG. 8also shows that the mass 10 includes openings 19. Such openings areprovided so that it is easier to etch the material beneath the mass 10,as will be understood more clearly with reference to the manufacturingprocess to be described later. The mass 10 is connected with anchors 50via springs 40.

There are many variations of the layouts, other than that shown in FIG.8. FIGS. 9 and 10 show two more layout embodiments. In the embodiment ofFIG. 9, some of the MEMS units are misaligned in at least one of the xand y directions, in x direction in this embodiment (the MEMS units 100have been rotated 90° as compared with FIG. 7, and therefore to beconsistent, we define the x dimension to be the dimension where theupper and lower electrodes 30 and 20 are misaligned with each other).The misalignment of the MEMS units 100 is for the purpose to limit thecontinuous length of the mass 10 in x, y or both directions (only in xdirection in this embodiment). More specifically, as shown in thefigure, the maximum length of the mass 10 is no longer x; the xdimension is divided into multiple segments x1, x2 and x3, each beingless than a predetermined length, e.g., 60 μm-100 μm. The shortenedcontinuous length in a direction helps to avoid bending in thatdirection. The length limit is applied only to the x dimension in thisembodiment, but it can certainly be applied to the y dimension as well,or both x and y dimensions. Note that the mass 10 in this embodimentalso includes openings 19.

FIG. 10 shows a layout embodiment which includes both features of “MEMSunit rotation” and “length limit”. The maximum length in y dimension islimited. This embodiment also shows that the MEMS units 10 do not haveto be of the same size. The springs and the anchors are not shown in thefigure; they may be located upper and lower sides of the mass 10 (in ydimension) or at the left and right sides of the mass 10 (in xdimension).

Hereinafter a manufacturing process according to the present inventionwill be explained with reference to FIGS. 11-14, taking the structureshown in FIG. 5 for example. A six-metal-layer process is shown as anexample, but the present invention can certainly be embodied in aprocess of any other number of metal layers, for making the structuresof FIGS. 1-10 and other structures under the spirit of the presentinvention.

Referring to FIG. 11, in this embodiment, a zero-layer wafer substrate71 is provided, which for example can be a silicon wafer so that theprocess is compatible with a standard CMOS process. Next, transistordevices can be formed by standard CMOS process steps as required (notshown), followed by deposition, lithography and etch steps to forminterconnection layers including a contact layer 72, metal layers 73,and via layers 74. The patterns of the interconnection layers are suchthat they form the upper and lower electrodes 30 and 20, and the mass 10(including the main body 13, the outer mass parts 11 and 15, and the twocapacitor plates 12 and 14); and an oxide region 75 (the “to-be-etchedregion”) is concurrently formed in the interconnection layers. In orderthat the etching of the oxide region 75 does not damage the other areasin the MEMS device, preferably, the oxide region 75 is encompassed in aguard ring 60, The guard ring can be formed concurrently with thepatterns of the layers 72, 73 and 74.

In the structure as shown, for example, the metal layers can be made ofaluminum; the contact layer and the via layers can be made of tungsten;and the oxide region 75 may be made of silicon oxide. The mass 10 andthe lower electrode 20 may be completely made by tungsten, or made byoxide 76 enclosed by surrounding tungsten, as shown in the figure. Ofcourse, the latter costs lower.

Referring to FIG. 12, a mask 77 is formed and patterned to expose theoxide region 75. The mask 77 for example can be a photoresist layerpatterned by lithography, or other materials such as a metal layer or anamorphous silicon layer.

Referring to FIG. 13, an oxide etch step is performed according to thepattern of the mask 77, to remove the oxide inside the narrow portionsof the oxide region 75. The etch for example can be anisotropic RIE(reactive ion etch).

Next referring to FIG. 14, another etch step is performed on the oxideregion 75 to remove it. The etch for example can be HF (hydrogenfluoride) vapor etch, or BOE (buffered oxide etch) by immersing thewhole wafer in an acid tank.

Finally, if the mask 77 is a photoresist layer, it can be removed; or,if the mask 77 is a layer of some other material, it can be kept orremoved as desired. A desired MEMS device is thus obtained.

Although the present invention has been described in detail withreference to certain preferred embodiments thereof, the description isfor illustrative purpose and not for limiting the scope of theinvention. For example, the openings in the mass and the layout of theMEMS units may be arranged in various other ways than that shown inFIGS. 8-10. As another example, the aluminum (for metal layers),tungsten (for contact and via layers) and silicon dioxide (for theto-be-etched region) may be replaced by copper and low dielectricconstant materials. One skilled in this art can readily think of othermodifications and variations in light of the teaching by the presentinvention. In view of the foregoing, it is intended that the presentinvention cover all such modifications and variations, which shouldinterpreted to fall within the scope of the following claims and theirequivalents.

1. A method for making a MEMS device, comprising: providing a substrate;depositing and patterning at least a contact layer, a plurality of metallayers and a plurality of via layers on the substrate, the contactlayer, metal layers and via layers as a whole include a to-be-etchedregion therein; and removing the to-be-etched region to form a MEMSdevice which includes: a mass including a main body and two capacitorplates located at the two sides of the main body and connected with themain body, the two capacitor plates being at different elevation levels;an upper electrode located above one of the two capacitor plates,forming one capacitor therewith; and a lower electrode located below theother of the two capacitor plates, forming another capacitor therewith.2. The method of claim 1, wherein the to-be-etched region is made of anoxide.
 3. The method of claim 1, wherein the step of removing theto-be-etched region includes: first etching the to-be-etched region byan anisotropic reactive ion etch, and then etching the to-be-etchedregion by hydrogen fluoride vapor etch or buffered oxide etch.
 4. Themethod of claim 1, wherein the upper and lower electrodes are misalignedwith each other in a horizontal direction.